U.S. patent number 5,492,587 [Application Number 08/223,983] was granted by the patent office on 1996-02-20 for method for using glass substrate surface modifiers in the fabrication of photochemically stable deep ultraviolet pellicles.
Invention is credited to Gilbert H. Hong.
United States Patent |
5,492,587 |
Hong |
February 20, 1996 |
Method for using glass substrate surface modifiers in the
fabrication of photochemically stable deep ultraviolet
pellicles
Abstract
A pellicle for use on a photomask reticle in conjunction with
deep ultraviolet light wavelengths. The membrane of the pellicle
comprises a purified fluoropolymer that has been spin coated on a
glass substrate treated with a surface modifier and then separated
and mounted on an aluminum frame. The aluminum frame has vents that
filter air of contaminants and which allow an equalization of air
pressure on both sides of the pellicle membrane when a peel-off
backliner is in place on the opposite side of the frame. A
permanent bond is made between the membrane and frame and a sticky
adhesive is used to keep the backliner on the frame until peel-off.
The sticky adhesive is such that the backliner may be re-attached a
plurality of times.
Inventors: |
Hong; Gilbert H. (Los Altos
Hills, CA) |
Family
ID: |
25469039 |
Appl.
No.: |
08/223,983 |
Filed: |
April 6, 1994 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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936758 |
Aug 21, 1992 |
5344677 |
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936758 |
Aug 21, 1992 |
5344677 |
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Current U.S.
Class: |
156/246; 156/241;
156/242; 156/74; 264/127; 264/216; 428/38; 430/5 |
Current CPC
Class: |
G03F
1/62 (20130101); G03F 1/64 (20130101); Y10T
428/31544 (20150401) |
Current International
Class: |
G03F
1/14 (20060101); B29D 007/01 () |
Field of
Search: |
;156/60,74,241,246,242
;430/5 ;428/38,630,13,14 ;264/128,216 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
"Surface Coating," Tappi, Dec. 1980, p. 175..
|
Primary Examiner: Simmons; David A.
Assistant Examiner: Helmer; Steven J.
Attorney, Agent or Firm: Schatzel; Thomas E.
Parent Case Text
COPENDING APPLICATIONS
This application is a division and a continuation-in-part of U.S.
patent application Ser. No. 07/936,758, filed Aug. 21, 1992, and
titled, PHOTOCHEMICALLY STABLE DEEP ULTRAVIOLET PELLICLES FOR
EXCIMER LASERS, now U.S. Pat. No. 5,344,677.
Claims
What is claimed is:
1. A method for forming a fluoropolymer film on a reusable
nitrocellulose substrate and then separating the fluoropolymer
film, the method comprising the steps of:
coating a nitrocellulose substrate on a super-polished
photomask-grade soda lime glass substrate such that both said
nitrocellulose substrate and said glass substrate are free of any
defects or contamination, said nitrocellulose substrate being a
film 0.5 to 3.0 micrometers thick and presenting a clean,
non-sticking surface to fluoropolymers;
spin coating a fluoropolymer film on top of the nitrocellulose
films to a thickness of approximately 0.5 to 3.0 micrometers;
bonding mending tape to said fluoropolymer film with a
fluoroadhesive;
attaching a stainless steel frame placed on top of said
glass-nitrocellulose-fluoropolymer-fluoroadhesive combination with
said mending tape; and
peeling off said fluoropolymer film from said nitrocellulose
substrate such that said fluoropolymer film remains attached to
said stainless steel frame.
2. A method for forming a fluoropolymer film on a reusable
substrate and then separating the fluoropolymer film, the method
comprising the steps of:
coating a substrate selected from the group of cellulose, acetate,
ethyl cellulose, cellulose acetate butyrate, polyvinyl butyral and
silicones on a superpolished photomask-grade soda lime glass
substrate such that both said substrate and said glass substrate
are free of any defects or contamination, said substrate being a
film 0.5 to 3.0 micrometers thick and presenting a clean,
non-sticking surface to fluoropolymers;
spin coating a fluoropolymer film on top of said substrate film to
a thickness of approximately 0.5 to 3.0 micrometers;
bonding mending tape to said fluoropolymer film with a
fluoroadhesive;
attaching a stainless steel frame placed on top of said glass
substrate fluoropolymer-fluoroadhesive combination with said
mending tape; and
peeling off said fluoropolymer film from said substrate such that
said fluoropolymer film remains attached to said stainless steel
frame.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to pellicles for protecting
photomasks from particulate contamination and more particularly to
pellicles suitable for use with deep ultraviolet radiation as is
present when using excimer lasers.
2. Description of the Prior Art
Pellicles are used as protective covers to keep particulate matter
outside of a focal plane of an optical apparatus so that a
desirable image is not disturbed. Pellicles generally comprise
thin, transparent membranes or films of polymer stretched over an
aluminum frame that is mounted to form a hermetically sealed,
dust-free enclosure over a photomask reticle. Pellicles are widely
used in semiconductor manufacturing of integrated circuit, both to
protect photomasks from particulate contamination and to extend the
mask life. Their major function is to eliminate soft defects and
improve die yield. The use of pellicles by semiconductor
manufacturers is reviewed in detail by Ron Iscoff in "Pellicles
1985: An Update," Semiconductor International (April 1985).
Projection printing systems also use pellicles, as is described by
Shea, et al., in U.S. Pat. No. 4,131,363, issued Dec. 26, 1978. A
broad class of pellicles and a method for forming these pellicles
is also described by Winn, in U.S. Pat. Nos. 4,378,953 and
4,536,240, issued on Apr. 5, 1983 and Aug. 20, 1985, respectively.
The following U.S. Pat. Nos. also describe pellicles: 4,973,142,
issued Nov. 27, 1990, to Edward N. Squire, and assigned to du Pont;
4,948,851, issued Aug. 14, 1990, to Edward N. Squire, and assigned
to du Pont; 5,008,156, issued Apr. 1, 1991, to Gilbert H. Hong, and
assigned to Exion; and 4,657,805, issued Apr. 14, 1987, to Y.
Fukumitsu, and assigned to Asahi Chemical.
Nitrocellulose has been widely used in pellicle manufacturing. But
nitrocellulose is not suitable for 248 nanometers (or shorter
wavelengths) lithography because nitrocellulose is highly absorbing
at 248 nanometers and will rapidly degrade. The use of
nitrocellulose has also declined because nitrocellulose is highly
flammable and must be stored in a wetted condition. Nitrocellulose
is also hygroscopic, which makes manufacturing under humid
conditions difficult. Thus as a finished product, pellicles of
nitrocellulose wrinkle when wetted with water, which makes cleaning
or storing under humid conditions a problem. A critical problem
with nitrocellulose pellicles is that the nitrocellulose material
itself does not transmit ultraviolet (UV) light well enough for use
in modern equipment that depend on the use of deep ultraviolet
light. Irradiation with ultraviolet light can also cause a
nitrocellulose pellicle membrane to become discolored, thus
reducing its transparency. Below two hundred and sixty nanometers,
even non-discolored nitrocellulose transmits less than seventy
percent (70%) of incident light. This limitation in nitrocellulose,
and also in MYLAR, when used in pellicles, is discussed by R.
Hershel, in "Pellicle Protection of IC Masks," A Report by Hershel
Consulting, Inc. (August 1981).
Advances in lithographic processes used in manufacturing integrated
circuits depend on reducing the wavelength of the incident
ultraviolet light used in conjunction with pellicles. The
development of a broadband pellicle capable of transmitting
ultraviolet light is described by I. E. Ward and D. L. Duly, in
"Optical Microlithography III: Technology for the Next Decade,"
SPIE, Vol. 470, pp. 147-154 (H. L. Stover, Editor), 1984. Ward and
Duly describe an antireflective layer that is coated on at least
one side of a pellicle, in order to reduce any optical
interference. Such optical interference is typically caused by
internal reflections of light within a pellicle and is evidenced by
an oscillating behavior in the transmission spectrum of a pellicle.
Proposed solutions to this particular problem have included
applying antireflective coatings and controlling the thickness of
the membrane. Antireflective coatings do not adhere well to a
pellicle's surface. The imperfect adhesion often then results in
cracking and flaking of the antireflective coating, thus ruining
the pellicle.
U.S. Pat. No. 4,657,805, issued Apr. 14, 1987, to Fukumitsu, et
al., discloses the use of thin fluoropolymer films to serve as
antireflective layers for a pellicle. Multiple layers of the
fluoropolymer films are coated on a core layer pellicle to form a
five-layer pellicle structure with the indexes of refraction of the
various layers being chosen to reduce internal reflection and
scattering.
The ultraviolet transmitting pellicles of Ward are described more
completely in a series of three patents. U.S. Pat. No. 4,482,591,
issued Nov. 13, 1984, discloses a pellicle comprised of polyvinyl
butyral resin (PBR) and the use of a ring with an adhesive side to
remove the pellicle from a wafer. U.S. Pat. No. 4,499,231, issued
Feb. 15, 1985, discloses a pellicle comprising PBR and a dispersion
of colloidal silica. U.S. Pat. No. 4,476,172, issued Oct. 9, 1984,
discloses pellicles comprised of a PBR derivative that includes a
silane moiety.
Problems also exist in the processes used to manufacture pellicles.
For example, typically, a pellicle is formed by depositing a
polymer solution on an inert substrate and then evaporating the
solvent. This leaves the pellicle coated on the inert substrate.
Removing the delicate pellicle from the substrate is a difficult,
but a necessary step in the process. U.S. Pat. No. 4,536,240,
issued to Winn, discloses a method for accomplishing this task by
bonding a frame to the pellicle and then peeling the pellicle off
the substrate. In conjunction with this procedure, a suitable
release agent can be applied to the substrate prior to applying the
fluoropolymer solution and thus aid in removing the pellicle. This
procedure, however, results in a high number of pellicles being
ripped during the removal step.
Duly, et al., in U.S. Pat. No. 4,523,974, issued Jun. 18, 1985,
disclose a method for manufacturing a pellicle from
polymethylmethacrylate (PMMA) that includes the steps of applying a
gold film to the surface of an oxidized wafer, coating a thin layer
of PMMA on the gold film, removing the PMMA and gold layers from
the wafer and etching off the gold layer.
Microlithography trends for the last decade have been towards
shorter and shorter wavelengths of ultraviolet radiation. The
stepper radiation is changed from mercury G-line of 436 nanometers
to I-line of 365 nanometers. A state-of-the-art stepper utilizes
krypton fluoride emission at 248 nanometers and XE-F at 194
nanometers to delineate feature sizes around 0.3 micron.
Pellicles are well accepted by the photomask industry as an
effective means of protecting the cleanliness of masks used in
microlithographic processes. When masks are pelliclelized and used
in transferring images of IC design on mask to wafer, the pellicles
serve not only as a protective dust cover, but also as a part of
the optics that do the lithographic imaging. Pellicle membranes
must be photochemically stable to deep ultraviolet radiation, e.g.,
to wavelengths of 194 nanometers and 248 nanometers for excimer
laser steppers. Pellicle membranes must be highly transparent into
the deep ultraviolet range to guarantee high wafer throughput.
Pellicle membranes must be very clean to ensure that no defects
result in the wafers being processed. Pellicle membranes must be
able to attach to an aluminum frame with appropriate adhesives and
be strong, even at the typical thickness of 0.5 micrometers to 5.0
micrometers, to ensure the assembled pellicles are stout enough for
ordinary use.
Although fluoropolymers have been described in the prior art as
useful for deep ultraviolet pellicles with aluminum frames, a need
nevertheless exists for a high yield method of casting the
fluoropolymer films, a pellicle for eliminating bursting due to
trapped air when the atmospheric pressure changes and a backliner
that cooperates with robotics used in automated manufacturing
facilities.
It is understood by those skilled in the art that pellicle
manufacturing involves the coating of a glass substrate and a later
peeling-off of a resulting membrane. Manufacturing yields are
reduced by the tendency of the membrane to adhere too strongly to
the glass substrate. In particular, this is a problem in the
manufacture of deep ultraviolet (DUV) pellicles made of
fluoropolymers.
SUMMARY OF THE PRESENT INVENTION
It is therefore an object of the present invention to provide a
method of casting the fluoropolymer films.
It is a further object of the present invention to provide a
pellicle frame that vents differences in atmospheric pressure to
eliminate bursting in the pellicle membrane.
It is another object of the present invention to provide a pellicle
with a backliner that is compatible with robotics used in automated
manufacturing facilities.
Briefly, an embodiment of the present invention is a pellicle with
a fluoropolymer membrane that has been spin coated on a
nitrocellulose substrate before separating, a vented aluminum frame
to which the separated fluoropolymer has been permanently bonded,
and a stiff peel-off backliner that is secured to the aluminum
frame with a sticky adhesive that permits multiple cycles of
detachment and reattachment of the frame to backliner.
An advantage of the present invention is that it provides a
pellicle that is economical to manufacture because very few of the
fluoropolymer membranes are damaged during separation from their
substrates.
Another advantage of the present invention is that it provides a
pellicle that will not burst in or out due to changes in
atmospheric pressure.
A further advantage of the present invention is that it provides a
pellicle that is compatible with robotics used in automated
manufacturing facilities.
These and other objects and advantages of the present invention
will no doubt become obvious to those of ordinary skill in the art
after having read the following detailed description of the
preferred embodiments which are illustrated in the various drawing
figures.
IN THE DRAWINGS
FIG. 1 is a perspective exploded assembly diagram of a pellicle and
shipping container of the present invention;
FIG. 2A is a perspective cutaway diagram of the vent in a part of
the frame of the pellicle of FIG. 1;
FIG. 2B is atop elevation view of the vent of FIG. 2A;
FIG. 2C is a side elevation view of the vent of FIG. 2A; and
FIG. 3 is a cross-sectional diagram of the fluoropolymer membrane
of the pellicle of FIG. 1 before being separated from the
nitrocellulose on glass substrates on which it is formed by spin
coating.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
FIG. 1 illustrates an embodiment of a pellicle of the present
invention, referred to by the general reference numeral 10. The
pellicle 10 comprises an amorphous fluoropolymer membrane 12, an
aluminum frame 14 including vents 16 and a backliner 18 with a
thumb tab 20 and an adhesive sticker 21. Acceptable fluoropolymer
materials useful for membrane 12 include tetrafluoroethylene
(TFE)-hexafluoropropylene (HFP) copolymer, vinylidene fluoride
homopolymer, vinylidene fluoride (VdF)-TFE copolymer,
TFE-perfluoroalkyl vinyl ether copolymer and VdF-ethylene
copolymer. Preferred fluoropolymers are dissolvable in solvents and
formable into uniform films by the spinner coating method. Pellicle
10 is intended for use in applications with 193 nanometers eximer
laser radiation. The materials mentioned herein for membrane 12 are
suitable for such use. Numerous application environments comprise
ultraviolet radiation at 248 nanometers, and the materials
mentioned herein for membrane 12 are also suitable for such use.
Since such radiation may degrade membrane 12, the preferred
thicknesses for membrane 12 ranges from a low of 0.5 microns to a
high of three microns.
Du Pont has developed at least two fluoropolymer materials
pertinent to pellicle manufacture and has made them commercially
available. For example, see U.S. Pat. No. 4,973,142, issued Nov.
27, 1990, and U.S. Pat. No. 4,948,851, issued Aug. 14, 1990, both
to Edward N. Squire. A first amorphous fluoropolymer is marketed by
Du Pont as "AF-2400", which is a copolymer of
perfluoro-2,2-dimethyl-1,3-dioxide (PDD) and tetrafluoroethylene
(TFE), with 85% PDD and 15% TFE. The glass transition temperature
of this copolymer is 240.degree. C. A second amorphous
fluoropolymer is sold as "AF-1600", which is a copolymer of PDD and
TFE with 65% PDD and 35% TFE. The glass transition temperature of
this latter copolymer is 160.degree. C. Generally, the higher the
concentration of PDD, the higher will be the rigidity and the glass
transition temperature. Both of these copolymers are soluble in 3M
Company FC FLUORINERT liquids (e.g., FC 72 which has a boiling
point of 56.degree. C., and FC 40 which has a boiling point of
155.degree. C.). AF-1600 can be prepared in a mixture of FC-72:
FC/40 (1:3) to a concentration of 8-10% and AF-2400 to 2-3%.
AF-1600 has exhibited adequate solubility to form acceptable
pellicle membranes. Ashahi Glass (japan) markets a
per-fluoro-cyclo-oxy-aliphatic-homopolymer under the name "CYTOP",
which has a glass transition temperature of 108.degree. C. and is
more soluble in per-fluorited liquids. However the films it forms
are not as rigid as the ones made of AF-1600 and AF-2400.
Nevertheless, AF-1600 and CYTOP are suitable for application
environments comprising ultraviolet radiation in the range of
190-500 nanometers.
For more information regarding such fluoropolymers, refer to U.S.
Pat. No. 4,657,805, issued Apr. 14, 1987, to Fukimitsu, et al. For
more information regarding spin-coating, refer to U.S. Pat. No.
4,536,240, issued Aug. 20, 1985, to R. Winn.
Frame 14 is typically 3.75 inches by 4.75 inches. The perimeter
material of frame 14 is approximately 0.25 inches by 0.0625 inches.
Vents 16 allow air but not contamination to pass through frame 14
such that changes in atmospheric pressure will not tend to burst
membrane 12 in or out when both membrane 12 and backliner 18 are
sealed to frame 14 or when pellicle 10 is in use on a photomask
reticle. A high-strength, ultraviolet-curable cement is preferably
used to permanently seal membrane 12 to frame 14. A tacky,
restickable type adhesive is used to form a temporary seal between
frame 14 and backliner 18 to allow multiple cycles of peel-off and
reattachment of backliner 18 to frame 14. (Such an adhesive is
familiar to lay persons as that used in the yellow 3M Company
POST-IT note pads.) Sticker 21 is color-coded differently on each
side, e.g., one side gold and the other side silver in color. Such
color-coding assists a user in consistently returning the same side
of backliner 18 to frame 14.
Pellicle 10 is preferably transported inside a shipping container
bottom 22 which includes a pair of finger notches 24 and a pair of
thumb tab notches 26. Pellicle 10 rests on a ledge 28 when
deposited within shipping container bottom 22. A clear, see-through
plastic shipping container cover 30 fits over shipping container
bottom 22 such that pellicle 10 may be seen by a user and protected
within during transport. The shipping container bottom 22 and cover
30 are such that they are compatible with robotic manipulators that
can open cover 30, extract pellicle 10 and separate backliner 18
from frame 14. Backliner 18 is also stiff enough to allow a robotic
vacuum lifter to lift away backliner 18 from the adhesive on frame
14. Such stiffness permits an automated system to remove backliner
18 without folding, wrinkling or stretching backliner 18. Pellicle
10 is then attached by robotic manipulation to a photomask reticle.
Backliner 18 guards the air volume within frame 14 and behind
membrane 12 from contamination until final mounting on the
photomask, which is preferably done in a cleanroom environment.
This function is especially important because it is the area
between the photomask and membrane 12 that will be in the focal
plane of the optics associated with the photomask. Any dirt or
contamination within the focal plane will create optic anomalies
that cause wafer processing defects. Any dirt or contamination
outside pellicle 10, and therefore outside the focal plane, cannot
be focused sharp enough to cause such defects.
FIGS. 2A, 2B and 2C illustrate the construction of vent 16 which
comprises a pair of holes 40 and 42 and a channel 44 within
sidewall 14. Preferably, hole 40 and hole 42 are placed near
opposite ends of the channel 44 such that air flow through vent 16
must pass through the length of channel 44. Holes 40 and 42 are
therefore not straight through the wall 14, but rather are offset
with each hole leading into the channel 44, one from inside the
pellicle cavity and the other from the outside environment. Holes
40 and 42 are typically of 0.2-0.6 millimeters inside diameter and
channel 44 has an inside width of 0.7 millimeters. The length and
depth of channel 44 may be tailored to fit a filter material 46
(FIGS. 2B and 2C only) that may alternatively be disposed within
channel 44. The inside geometry of channel 44 may be modified to
gain certain benefits. For example, increasing the length of
channel 44 may enhance the particle trapping capability,
particularly if the adhesive used between frame 14 and backliner 18
forms one boundary of channel 44. Particles may also be blocked by
shaping channel 44 in a zig-zag fashion. The air velocity of
particles within channel 44 may also be decreased by shaping
channel 44 like a horn. Tests by the inventor indicate that
pellicle 10 when placed in the cargo bay of a high altitude
commercial airliner will be able to bleed-off pressure
differentials through vent 16 fast enough to cope with the normal
rate of climb and descent of such aircraft before membrane 12 will
be injured. The material selected for filter 46 must support high
flowrates at the necessary pore sizes. Good tensile strength and
resistance to fracture under lateral stresses is also important.
Filter 46 may comprise a 3.0 micrometer filter of
polytetrafluoroethylene, a hydrophobic material immune to wetting
by the absorption of moisture. Fluoropore Company makes such filter
material and sells it commercially. Filter 46 is attached to frame
14 with an adhesive, e.g., 3M Company adhesive 4952, such that the
walls are at least one millimeter thick. The adhesive should be
allowed to cure for at least thirty minutes after use before
pellicle 10 is washed.
Pellicle membranes 12 are fabricated on a reusable nitrocellulose
substrate according to the steps listed in Table I.
TABLE I ______________________________________ STEP DESCRIPTION
______________________________________ 1 Depositing a film of
nitrocellulose on a super-polished photomask-grade soda lime glass
substrate. Both the nitrocellulose and glass substrate must be
substantially free of any defects or contamination. The
nitrocellulose deposited in a film that is approximately 0.5 to 3.0
micrometers thick, and should present a clean, non-stick surface. 2
Depositing a film of fluoropolymer, such as CYTOP or amorphous
fluoropolymers 1600 or 2400, to a thickness of approximately 0.5 to
3.0 micrometers over the nitrocellulose film. A well-tuned spinner
may be used to obtain a such a film thickness. 3 Bonding perimeter
edges of the fluoropolymer film to pieces of mending tape with a
fluoro- material, e.g., KEL-F-800. 4 Attaching with the mending
tape a stainless steel frame on top of the stack of glass,
nitrocellulose, fluoropolymer, and fluoro- adhesive films. 5
Peeling off the combination of film, mending tape and stainless
steel frame. 6 Attaching only the fluoropolymer film of the
combination of film, mending tape and stainless steel frame to an
aluminum frame. 7 Trimming away the stainless steel frame and
mending tape from outside the perimeter of the aluminum frame such
that a taut fluoropolymer film is stretched over the aluminum
frame. ______________________________________
The first four steps of the process described in Table I results in
the structure illustrated in FIG. 3 which comprises a soda lime
glass substrate 50, a nitrocellulose intermediate layer 52, a
fluoropolymer layer 54, a fluoro-adhesive layer 56, a mending tape
layer 58 and a stainless steel frame 60. After a lifting-off with
mending tape layer 58, layers 54, 56, 58 and frame 60 stay together
and separate from layer 52, which remains on the glass substrate 50
and is typically left in a condition good enough to be reused. The
prior art methods differ from the present invention in at least two
ways. First, in the present invention, glass substrate 50 is coated
with the nitrocellulose layer 52 before the additional layer of
fluoropolymer 54 is applied. This takes advantage of the fact that
the fluoropolymer layer 54 separates rather easily at the
nitrocellulose-to-fluoropolymer interface. If fluoropolymer layer
54 were put directly on to glass 50, the glass-to-fluoropolymer
interface would be very difficult to separate, and the film would
probably tear in the attempt. The prior art solved such problems
with the use of various solvents. Second, fluoro-adhesive 56 is
applied to the fluoropolymer 54 such that the film can be lifted
using a combination of mending tape 58 and stainless steel frame
60. The mending tape may be SCOTCH.TM. brand Magic Tape.TM. 810, as
sold by 3M Commercial Office Supply Division (St. Paul, Minn.
55144-1000). Fluoropolymer 54 is then transferred to an aluminum
frame, such as frame 14 (FIG. 1) and becomes what is identified as
membrane 12 in FIG. 1 after trimming away stainless steel frame
60.
The method of fabricating pellicle membrane 12, described herein,
is related to that described in U.S. Pat. No. 5,008,156, issued
Apr. 16, 1991, to Gilbert H. Hong, the present inventor, and which
patent disclosure is incorporated herein by reference. In general,
some prior art methods start by preparing a coating solution of
polymer in a solvent. This solution is filtered through a filter
with a pore size of 0.2 micron, or smaller, in order to remove
contaminants, and thus improve the clarity of the finished film. A
class one hundred (or better) clean room is used during the
pellicle manufacture to avoid airborne contamination. After
filtering, the fluoropolymer solution is applied to a super-flat,
smooth and defect-free substrate, using conventional spin-coating
techniques. (Spin-coating is widely used in the semiconductor
industry for obtaining polymer films and most of the accepted
industry practices for obtaining defect-free coatings can be
adapted to pellicle manufacturing.) Typically, a speed of about one
thousand RPM is used to obtain a high quality two to three micron
pellicle. Substrates of soda-lime glass are preferred. Prior to
use, the glass substrate is cleaned by scrubbing in deionized water
with detergent. The substrate is then rinsed with copious amounts
of deionized water. This is followed by a second rinse with
isopropyl alcohol using ultrasonic agitation. The substrate is
dried in a degreaser tank with a chlorinated fluorocarbon, e.g.,
FREON. In consideration of the environment, other types of chemical
degreasers may be preferred. After spin-coating, any residual
solvent is removed from pellicle membrane by heating in a
super-clean oven for about thirty minutes at approximately
60.degree. to 90.degree. C. This heating serves to increase
membrane tensile strength by reducing stresses in the
fluoropolymer. Additional layers may be added through additional
spin-coating steps. In certain applications, and after drying,
pellicle membrane is peeled-off of the glass substrate prior to
being mounted to a square frame. This improves the yield of
pellicle membranes by reducing membrane breakage. Typically, the
square frame is made of stainless steel and has a thickness of
about twenty mils and an inside dimension of six to seven inches,
depending on the size of pellicle membrane. In general, the square
frame is attached to pellicle membrane by adhesive strips, such as
3M SCOTCH brand Magic Tape. A membrane assembly, with the glass
substrate on one side and the square frame on the other, is
submerged into or sprayed with deionized water for about five
minutes. An adhesion failure at the interface of the pellicle
membrane and glass is induced by the parting action of the
deionized water. The pellicle membrane separates from the glass
substrate and yet remains attached to square frame. Either gentle
heating or ambient evaporation heating will remove water droplets
from the surface of the pellicle membrane. After the peel-off and
drying, the pellicle membrane can be transferred to a frame. This
is done by placing the square frame containing the pellicle
membrane on top of the frame to which a permanent adhesive has
previously been applied. After the permanent adhesive has hardened,
the pellicle membrane is separated from the square frame by
trimming away excess membrane material along the frame to yield the
edge-mounted pellicle membrane. In typical applications, the
thickness of pellicle membrane is chosen to be either 0.85 microns
or 2.83 microns. In some variations, the thickness of pellicle
membrane is varied to reduce optical interference effects or to
adjust the strength of the membrane.
Fluoropolymers can generally be dissolved in a perfluorinated
fluid, such as FLURINERT by 3M Company. However, the
fluoro-adhesives preferably comprise material that is soluble in
traditional solvents, e.g., methyl-ethyl ketone, ethoxy-ethyl
acetate or propylene-methyl acetate. Good results have been
obtained with KEL-F 800 (3M Company), and KYNAR 7201 and 9301 by
Pennwalt have been proven to give acceptable results. The second
layer must be dissolvable by a solvent that will not attack the
first previously-spun layer.
Multiple-antireflective coated deep ultraviolet pellicle membranes
12 can be made by using a structure comprising a first
fluoropolymer layer, a second fluoroadhesive layer with high-index,
e.g., 1.435, and a third fluoropolymer layer. For example, the
first layer comprises AF-1600, the second layer comprises KEL-F,
and the third layer comprises AF-1600.
Two types of experiments were performed by the inventor. A
relaxation experiment involved swelling of the membrane a distance
dx, and then measuring the time (dt.sub.return) required for dx to
approach zero. A constant dx, or venting experiment involved
pressurizing pellicle membrane 12 until it swelled to a given dx,
measuring the differential pressure (dP), and then timing how long
(dt.sub.vent) the membrane 12 can be held at dx by controlling the
rate of pressure drop through a vacuum valve. In this experiment
the pellicle membrane 12 cavities are often nearly completely
evacuated. The experiments were conducted in a non-cleanroom
environment, in order to simulate a worst case scenario in terms of
membrane efficiency and possible loading under severe conditions.
The relaxation data provides a rough estimate of the average
flowrate from a filtered pellicle membrane 12 whose vent hole is so
restricted that venting times would be prohibitively long to
measure. The venting data allows one to calculate the average
flowrate through the vent hole at a given constant pressure as well
as the rate of pressure drop needed to maintain this equilibrium.
Four different pellicle membranes 12 have been used: an Exion
Technology, Inc. (San Jose, Calif.) NI-108-63-B-G with a three
millimeter diameter hole for vent 16, an Exion Technology
PE-107-31-A-T with a groove and four 0.5 millimeters diameter holes
for vent 16, an Exion Technology PB-107-31-A-T without a groove and
with four holes of 1.0 millimeters, 0.7 millimeters, 0.7
millimeters, and 0.2 millimeters diameters for vents 16, an Exion
Technology PE-107-31-A-T with two recessed holes with an inner
diameter of 0.6 millimeters for vent 16, and an Exion Technology
CA-122V-40-B-T.
Three types of adhesives have been used: 3M Company 447 (ten mil
rubber-based), Norwood KC8031 (thirty mil), and 3M Company 4952 (45
mil, acrylic-based). The latter two adhesives were cut into
rectangular shapes with holes for vents 16 of approximately
0.07".times.0.018", with a minimum side wall thickness of 0.7
millimeters. This will increase the usable filter area, since the
filter membrane may bottleneck the air flow.
Five different Millipore membrane filters have been used for filter
46 (FIGS. 2B and 2C): a 0.2 micrometer pore size Fluoropore (PTFE
with a high density polyethylene backing), a 0.5 micrometer
Fluoropore, a 1.0 micrometer Fluoropore, a 3.0 micrometer
Fluoropore (PTFE with a polypropylene backing), and a 8.0
micrometer MF-Millipore (mixed esters of cellulose). All of the
membranes were cut to the appropriate size from larger discs.
Pellicle 10 preferably can withstand stowage while being
transported in the non-pressurized cargo hold of a commercial
airplane climbing 10000 feet in three minutes. This creates an
average rate of air pressure drop of 0.058 inches of mercury per
second (" Hg/s). For pellicles 10, a dP of less than 1" Hg has been
observed, which will cause less than 0.5 millimeters of deflection
of membrane 12. The potential pressure drop rate could be as high
as 0.067" Hg/s, however most cargo planes are at least partially
pressurized and the probable rates are much less.
For vent holes 40 and 42 there may be no real optimal hole size,
since micron pore size membrane filters restrict air flow by
several percent of the uninhibited flow. The hole size of holes 40
and 42 should be as large as possible, leaving approximately at
least one millimeter of space on frame 14 around holes 40 and 42
for adhesive. Alternatively, the size of holes 40 and 42 can be
kept relatively small while the number of holes can be increased in
compensation.
Of the many membrane filters that Millipore manufactures, only the
MF-Millipore and the Fluoropore series were found to have met the
necessary requirements for pore size and high flowrate. The
MF-Millipore filters do not appear to be suitable for this
application because they have insufficient tensile strength and
fracture under very small lateral stresses. The Fluoropore filters,
on the other hand, can stretch without tearing. In addition, they
are made of polytetrafluoroethylene, a hydrophobic material, and
are therefore immune to wetting by the absorption of moisture.
Depending on the pore size, wetted membranes can require from one
to twenty pounds per square inch (PSI) of pressure difference to
clear. Of the three Fluoropore filters tested, the 3.0 micrometer
filter performed the best, in terms of realizable flowrate. Actual
production using this filter may, however, pose several problems,
e.g., the polypropylene support side of the filter is fibrous and
sheds when abraded, the stiff PTFE surface adheres only weakly to
both the Norwood and 3M Company 4952 adhesives, and the filter 46
may peel off under stress. The adhesion is better with the other
three Fluoropore filters because of their malleability. The PTFE
side of the 3.0 micrometer filter also appears to be more fibrous
than the others, resulting in a tendency to be sometimes pulled
apart under stress. The 0.2 micrometer, 0.5 micrometer, and 1.0
micrometer Fluoropore filters are all supported by a web-like HDPE
backing. In most cases, this backing will not affect the flowrate,
but for small vent holes (0.1 millimeter radius) this material
should and can be carefully peeled off of the PTFE membrane.
Adhesive configurations have consisted of a six millimeters by five
millimeter piece of 3M Company 447 with a hole nearly the same size
as vent holes 40 and 42. Other tests changed to Norwood and 3M
Company 4952 with a larger, rectangular hole because the larger
available filter surface area partially un-bottlenecked the flow
and the greater thickness allowed the flow out of the orifice to
more fully develop, resulting in better usage of the available
surface area. There are a number of difficulties which must be
resolved in manufacturing. A principal concern is the precision
cutting of holes 40 and 42. For a pellicle 10 with a 3.1 millimeter
standoff, for example, the adhesive is 3.1 millimeters wide while
holes 40 and 42 are 1.7 millimeters ID. This permits walls of 0.7
millimeters on either side. With Norwood thirty mil (0.76
millimeters), the height of the walls is roughly equal to their
width. For 3M Company 4952, the walls are 0.4 millimeters higher
than they are thick. Both are spongy foam adhesives, and the result
is an extremely difficult material to cut cleanly and precisely.
After the adhesive is cut, then there may be additional problems
with the application of filter 46 precisely to the adhesive and
then the adhesive to frame 14. Other problems include possible
outgassing of 4952, the durability of the filter-to-adhesive bond
over time and the compatibility of a raised filter element with
existing pellicle handling and mounting fixtures.
Nevertheless, it appears that the best solution at present in terms
of flow is to include in vent 16 two to four large sized holes (0.4
millimeters to 0.5 millimeters) combined with the 3.0 micrometer
Fluoropore filter for filter 46 attached to frame 14 with 3M
Company 4952 adhesive cut such that the walls are at least one
millimeter thick. Since the bond strength of 3M Company 4952 is
supposed to build up in a logarithmic fashion over a 72 hour
period, the completed assembly of pellicle 10 allowed to sit
undisturbed for at least a half hour before washing. Possible
modifications and configurations:
In an alternative embodiment, vent 16 is replaced by a recessed
hole. A small circular filter is attached inside using either
double-sticky tape or epoxy glue such that the filter material is
flush with or below the surface of frame 14. This may resolve a
potential problem of compatibility with existing pellicle fixtures,
as well as relieving any concerns regarding the accidental removal
of filter 46 in vent 16 since the recessed filter cannot be brushed
out once it is attached inside the depression.
The present invention includes process schemes that use an adhesion
promotor, a fluoropolymer, a surface modifier and a glass
substrate. The surface modifier acts as a release agent to
facilitate the separation of the fluoropolymer membrane that has
filmed on the glass substrate. The adhesion promoter is used to
enhance the adhesion of the film membrane to the frame.
Surface modifiers are, in general, regarded as release agents.
However, in context with the present invention, such surface
modifiers are defined as those having any affect on the surface.
The inventor's research has demonstrated that it is sometimes
necessary to promote adhesion so that additional layers of material
may be deposited. Therefore, a surface modifier preferably
optimizes the adhesion for additional deposited films, but not so
strongly that yields are severely comprised by problems in
peeling-off the whole film.
Surface modification research conducted by the present inventor
with deposits of thin organic films with surface modifiers such as
nitrocellulose has led to the following conclusions. The glass
surfaces of the substrate should be modified by depositing a
suitable surface material that expresses an affinity for glass that
exceeds its affinity for fluoro-film. The resulting structure
allows the fluoro-film to be separated from the surface modifier
which stays with the glass substrate and can therefore e reused The
adhesion of the fluoro-films with the surface modifier must be
optimized to produce an affinity just great enough to support spin
coating. Otherwise, a coating of fluoro-film would be prevented.
However, that affinity cannot be so great as to compromise the
later peeling-off of the film. The surface modifier must also be
substantially pure. Filtering the surface modifier helps to form
near perfect thin films with an optical smoothness that is as good
as that of the surface of the glass substrate.
In particular, the research of the present inventor has led to the
use of nitrocellulose, cellulose acetate, ethyl cellulose,
cellulose acetate butyrate, polyvinyl butyral (PVB) and silicones,
as surface modifiers. Acceptable silicones include those
manufactured in the United States by General Electric (GE) as
UV9300, UV9310C, the SL6000 series and the SL5000 series. Curing is
done with heating or ultraviolet light, as directed by the
manufacturer.
Therefore, with reference to FIG. 3, the nitrocellulose
intermediate layer 52 can be substituted by materials which include
nitrocellulose, cellulose acetate, ethyl cellulose, cellulose
acetate butyrate, polyvinyl butyral (PVB) and silicones.
Fluoropolymer 1(AD) and 2(AS) appear to work the best with the
ultraviolet-cured type of silicones. Fluoropolymer 2(AS) appears to
works well with the PVB. In general, many pairs of film material
and surface modifiers are possible.
Although the present invention has been described in terms of the
presently preferred embodiments, it is to be understood that the
disclosure is not to be interpreted as limiting. Various
alterations and modifications will no doubt become apparent to
those skilled in the art after having read the above disclosure.
Accordingly, it is intended that the appended claims be interpreted
as covering all alterations and modifications as fall within the
true spirit and scope of the invention.
* * * * *